The present invention relates to transmitters of the type used in the process control industry which include a sensor for sensing a process variable. More specifically, the present invention relates to an improved sigma-delta analog to digital converter for converting an analog signal from such a sensor into a digital value.
Transmitters in the process control industry typically communicate with a controller using the same two wires over which they receive power. A transmitter receives commands from a controller and sends output signals representative of a sensed physical parameter (i.e., a process variable) back to the controller.
The transmitter includes a sensor for sensing the process variable. The sensor provides an analog output signal which is representative of the process variable. Such process variables include pressure, temperature, flow, pH, turbidity, gas concentration, etc. Many process variables have a very large dynamic range, for example, in some signals the amplitude can range by a factor of 10,000.
An analog to digital converter in the transmitter converts the analog sensor signal to a digital representation of the sensed physical parameter for subsequent analysis in the transmitter or for transmission to a remote location. Typically, a microprocessor in the transmitter compensates the digitized signal and an output circuit and the transmitter sends an output representative of the compensated physical parameter to the remote location. Although the digitized signal may be updated only a few times per second depending on the nature of the process to be controlled, the converter is frequently required to have, for example, 16 bits of resolution and be substantially insensitive to noise. Sigma-delta converters are used as analog to digital converters in transmitters. One embodiment of such a converter is a charge balance converter in which a capacitor is charged to a positive or negative potential and the resulting packets of charge are accumulated in an integrator. The charge is transferred onto a capacitor in the circuit by semiconductor based switches. The accumulated charge, which is representative of the sensed parameter, is compared to a reference level and the resulting output is used as a feedback signal to control the switch action, and therefore, the accumulation of positive and negative charge packets in the integrator. A digital representation of the sensed parameters is determined by counting the number of switch actuations.
One limitation of prior art analog to digital converters is that in order for them to be functional over a wide dynamic range, resolution must be sacrificed. This is particularly problematic, for example, with process variable sensors in which the sensor signal output has a large DC value which varies slowly with time, and a small AC signal which varies more rapidly. Thus, if it is necessary to measure the small AC signal to a specific accuracy, 0.1% for example, then the resolution of the analog to digital converter required to achieve this accuracy is not just the 10 bits of resolutions required to result to 0.1%. The resolution must be much higher due to the DC offset term which must also be accommodated in the A/D dynamic range.